1. Field of the Invention
This invention relates to a method for preparing a carbon/carbon composite material.
2. Prior Art
Carbon fiber reinforced carbon matrix composite materials are light in weight, have thermal resistance and, further, they are superior in sliding movability, strength, breaking toughness and thermal conductivity etc., so that they are used as industrial materials such as heat resistant materials, brake materials and furnace materials.
In the preparation of the carbon fiber reinforced/carbon composite materials, first carbon fibers are molded or carbon fibers together with pitch or a thermoset resin as a binder are molded, they are then carbonized to make preforms and the preforms so made are subsequently densified.
Although there has been proposed a method for completing said preform and densification in one step, the obtained carbon fiber reinforced carbon matrix composite materials will be insufficient in strength and sliding movability and will be unable to be used as heat resistant structural materials for space applications or brake materials even if they may be used as furnace materials and the like. Therefore, carbon fiber reinforced carbon matrix composite materials adapted for applications requiring sufficient strength and sliding movability are generally prepared by first making preforms thereof and then densifying the preforms.
A CVD (chemical vapor deposition) method has hitherto been most widely used as a process of densifying such preforms, but this method takes a long period of time and incurs high costs. On the other hand, there is a method for impregnating preforms with a thermosetting resin such as a phenolic resin or a furan resin and carbonizing the resin so impregnated, but this method also takes a long period of time for carbonization of the resin and is not preferable in view of the cost because of its low yield, and the method has a disadvantage that the obtained composite material is low in thermal conductivity.
Furthermore, there is a method which comprises using pitch for densification, but this method generally has disadvantages because pitch is carbonized via its liquid phase so as to be bubbled during carbonization by gases produced by thermal cracking, etc. at normal pressure with the result that the obtained carbonized material is markedly low in bulk density. Additionally, the yield of carbonized material obtained at this time is also low. In order to prevent bubbling during carbonization and a decrease in yield of carbonized material, there has been proposed a method for carbonization under pressure. For example, there has been reported in ICCM (International Conference on Composite Materials)-2, PP. 1302-1319 (1978) a method to carbonize pitch in a HIP device under a pressure of 6.9-68.9 MPa (70-703 kgf/cm.sup.2), whereby the yield obtained is improved.
It has also been reported in "Carbon. Vol. 11, PP. 570-574 (1973)" that yields of carbonized materials obtained by carbonizing pitch under pressure up to 100 bars (102 kgf/cm.sup.2) were examined whereby the pressuring effect was found to be achieved at a pressure of 25 bars (25.5 kgf/cm ) or above.
Further, "Carbon, No. 125, p.62 (1986)" describes that it is possible to make constant the yield of carbonization under a pressure of 1 MPa. However, this publication does not consider a method of manufacturing a carbon/carbon composite material excellent in densification efficiency by carbonizing a preform impregnated with pitch under a pressure off 10 Kgf/cm.sup.2 or less.
There are known two different kinds of compression method as a pressure-sintering method of carbon preform, i.e., a uniaxial compression method and an isotactic compression method.
According to the uniaxial compression method, a carbon preform impregnated in advance with a carbonaceous pitch is introduced into a die, and then sintered in the die while being unidirectionally compressed with a rod. On the other hand, according to the isotactic compression method, a carbon preform impregnated in advance with a carbonaceous pitch is disposed in a pressure chamber, and then heated in the pressure chamber after the pressure chamber is filled and sufficiently pressurized with a pressurizing medium, preferably an inert gas, or while the pressure chamber is filled and increasingly pressurized with a pressurizing medium, thereby isotactically sintering the carbon preform.
There is a problem that due to the vaporization of low boiling point components within the carbon preform or due to the generation of thermally decomposed gases from the carbon preform during the pressure-sintering, the carbon preform becomes porous, thus lowering the bulk density of the sintered body.
According to the uniaxial compression method, in view of increasing the bulk density of the carbon preform, the gases inside the carbon preform are allowed to be pushed out through a space between the die and rod by compressing the carbon preform with a rod from one direction of the carbon preform. On the other hand, according to the isotactic compression method, the cell within the preform is not removed from the preform, but is minimized in size and confined within the preform by increasing the pressure of the pressurizing medium.
The isotactic compression method is advantageous in that the bulk density of the carbon preform can be easily increased by repeating the densification treatment, since the cell is simply minimized according to this method in contrast to the uniaxial compression method.
However, these proposed methods require special devices such as a HIP (Hot Isostatic Pressure) device, an autoclave or a metallic bomb and are low in productivity. In addition, these devices for applying high pressures have been a cause for various limitations on the manufacture of large-sized products.